An anti-theft system such as that illustrated in FIG. 4 has been commonly employed to prevent shop lifting, etc. at retail stores where compact disk cassettes, magnetic tape cassettes, clothing garments, etc. are displayed in such a manner as to be readily accessible to customers.
Such a conventional anti-theft system consists of a theft prevention gate 30 installed near an exit 40 of the store and an alarm unit 20 attached to an article 50. The theft prevention gate 30 incorporates a circuit board 31 and a transmission antenna (not shown), and the circuit board 31 is provided with a transmitting circuit (not shown) which transmits an alarm activation signal to the alarm unit 20.
As indicated in FIG. 5, the alarm unit 20 has a buzzer 22, a circuit board 23, a battery (not shown), etc., each of which is housed in a casing 21, and in the surface of which casing a plurality of alarm emitting holes 24 are provided and an alarm activation switch piece 25 is embedded. The buzzer 22, which is controlled by the circuit board 23, is designed to be activated when the alarm unit 20 is removed from the article 50 or passes through the theft prevention gate 30. As an alternative to providing the alarm unit 20 with the alarm activation switch piece 25, the alarm unit 20 may be attached directly to the article 50 by means of a wire, in which case when the alarm unit 20 senses that the wire has been removed or cut or passed through the theft prevention gate 30, the buzzer 22 is activated.
At a retail store, the article 50 with the alarm unit 20 affixed thereto is displayed on a rack. When a sales clerk sells the article 50 to a customer (s), he first sends a specified reset signal from an alarm deactivation device (not shown) to the circuit board 23 of the alarm unit 20 to set the alarm unit such that the buzzer 22 will not be activated, then removes the alarm unit 20 from the article 50 and hands the article 50 to the customer when payment is made.
On the contrary, when the alarm unit 20 is removed from the article 50 by a customer, the alarm activation switch piece 25 is also removed and the buzzer 22 is activated. Further, in a case that a customer leaves a store premises taking away the article 50 with the alarm unit 20 still attached thereto, the circuit board 23 of the alarm unit 20 receives an alarm activation signal from the transmission antenna of the theft prevention gate 30, in response to which the buzzer 22 is activated.
However, a checkout counter is usually at the back of a store, far away from the exit 40 of the store where the theft prevention gate 30 is installed and therefore, an alarm from the buzzer 22 that goes off at the exit 40 may not be readily audible from the checkout counter, especially in an environment where the back ground music is present or where many customers are present, etc.
With a view to solving the problem of the anti-theft device described above, a burglar alarm system such as illustrated in FIG. 6 is used as a supplementary device in which an alarm sensor device 10 provided with a microphone 11 is disposed near the theft prevention gate 30 or in a fitting room and the like where a customer could remove and destroy or conceal the alarm unit 20 so that an alarm sensor device 10 senses an alarm 12 from the alarm unit 20 and issues an alarm signal 13, which is sent through a wire or by wireless connection to a speaker 15 or lamp 16 disposed at a checkout counter 14 to alert store personnel at the checkout counter that the alarm has been activated.
Such a system as described above, however, suffers from a problem in that it may not be able to discriminate an external noise similar to that generated by an alarm, such as background music, ambient noise, etc. from a legitimate alarm or may mistake reverberations of an alarm for an external noise.